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SIP                                                          D.R. Worley
Internet-Draft                                                Bluesocket
Expires: May 10, 2008                                   November 7, 2007


      Session Initiation Protocol Service Example -- Music on Hold
                    draft-worley-service-example-00

Status of this Memo

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   This Internet-Draft will expire on May 10, 2008.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   The "music on hold" feature is one of the most desired features of
   telephone systems in the business environment.  "Music on hold" is,
   when one party to a call has the call "on hold", that party's
   telephone provides an audio stream (often music) to be heard by the
   other party.  Architectural features of SIP make it difficult to
   implement music-on-hold in a way that is fully compliant with the
   standards.  The implementation of music-on-hold described in this
   document is fully standards-compliant, but is simpler than the
   methods previously documented.



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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Technique  . . . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.1.  Placing a Call on Hold and Providing an External Media
           Stream . . . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.2.  Taking a Call off Hold and Terminating the External
           Media Stream . . . . . . . . . . . . . . . . . . . . . . .  4
     2.3.  Example Message Flow . . . . . . . . . . . . . . . . . . .  5
     2.4.  Alternative Example Message Flow . . . . . . . . . . . . . 13
   3.  Advantages . . . . . . . . . . . . . . . . . . . . . . . . . . 22
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 23
   5.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
     5.1.  Normative References . . . . . . . . . . . . . . . . . . . 24
     5.2.  Informative References . . . . . . . . . . . . . . . . . . 24
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 25
   Intellectual Property and Copyright Statements . . . . . . . . . . 26


































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1.  Introduction

   Within SIP[1]-based systems, it is desirable to be able to provide
   features that are similar to those provided by traditional telephony
   systems.  A frequently requested feature is "music on hold": The
   music-on-hold feature, when one party to a call has the call "on
   hold", that party's telephone provides an audio stream (often music)
   to be heard by the other party.

   Architectural features of SIP make it difficult to implement music-
   on-hold in a way that is fully compliant with the standards.  The
   purpose of this document is to describe a method that is reasonably
   simple and fully standards-compliant.






































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2.  Technique

   The essence of the technique is that when the executing UA performs a
   re-INVITE of the remote UA to establish the hold state, it extracts
   the answer SDP[2], and uses that as the offer SDP in a new INVITE to
   the external media source.  The external media source is thus
   directed to provide media directly to the remote UA.

2.1.  Placing a Call on Hold and Providing an External Media Stream

   1.  The user instructs the user's UA to put the conversation on-hold.

   2.  The user's UA sends a re-INVITE to the remote UA with SDP that
       declines to receive media.  This establishes the on-hold state
       (in that direction).

   3.  The remote UA responds 200 to the re-INVITE, and includes SDP
       giving its own listening address/port, which should indicate that
       it will not send media.

   4.  The user's UA composes and sends a new INVITE to the configured
       external music-on-hold (MOH) source.  The SDP in this request is
       largely copied from the SDP returned by the remote UA in the
       previous step, particularly regarding the provided listening
       address/port and codec numbers.

   5.  The MOH source responds 200 to the INVITE.

   6.  After this point, the MOH source generates RTP containing the
       music-on-hold media, and sends it directly to the listening
       address/port of the remote UA.  The UA maintains two dialogs (one
       to the remote UA, one to the MOH source), but does not see or
       handle the MOH RTP.

2.2.  Taking a Call off Hold and Terminating the External Media Stream

   1.  The user instructs the user's UA to take the conversation off-
       hold.

   2.  The user's UA sends a re-INVITE to the remote UA with SDP that
       requests to receive media.

   3.  When the remote UA responds 200 to the re-INVITE, the user's UA
       sends BYE on dialog to the MOH source.

   4.  After this point, the MOH source does not generate RTP and
       ordinary RTP flow is re-established in the original dialog.




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2.3.  Example Message Flow

   This section shows a message flow which is an example of this
   technique.  The scenario is: Alice establishes a call with Bob. Bob
   then places the call on hold, with music-on-hold provided from an
   external server.  Bob then takes the call off hold.

   Note that this is just one possible message flow that illustrates
   this technique; numerous variations on these operations are allowed
   by the applicable standards.

   Alice             Bob       Music Server

   Alice establishes the call:

     |                |              |
     |    INVITE F1   |              |
     |--------------->|              |
     | 180 Ringing F2 |              |
     |<---------------|              |
     |    200 OK F3   |              |
     |<---------------|              |
     |     ACK F4     |              |
     |--------------->|              |
     |       RTP      |              |
     |<==============>|              |
     |                |              |

   Bob places Alice on hold:

     |                |              |
     |INVITE (hold) F5|              |
     |<---------------|              |
     |    200 OK F6   |              |
     |--------------->|              |
     |     ACK F7     |              |
     |<---------------|              |
     |    no RTP      |              |
     |                |              |

   Bob's UA initiates music-on-hold:

     |                |              |
     |                |   INVITE F8  |
     |                |------------->|
     |                |   200 OK F9  |
     |                |<-------------|
     |                |    ACK F10   |



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     |                |------------->|
     |     Music-on-hold RTP         |
     |<==============================|
     |                |              |

   The music on hold is active.

   Bob takes Alice off hold:

     |                |              |
     |  INVITE F11    |              |
     |<---------------|              |
     |   200 OK F12   |              |
     |--------------->|              |
     |     ACK F13    |              |
     |<---------------|              |
     |                |    BYE F14   |
     |                |------------->|
     |                |    200 F15   |
     |                |<-------------|
     |       RTP      |              |
     |<==============>|              |
     |                |              |

   The normal media session between Alice and Bob is resumed.

   Message Details

    /* Alice calls Bob. */

    F1 INVITE Alice -> Bob

    INVITE sips:bob@biloxi.example.com SIP/2.0
    Via: SIP/2.0/TLS atlanta.example.com:5061
     ;branch=z9hG4bK74bf9
    Max-Forwards: 70
    From: Alice <sips:alice@atlanta.example.com>;tag=1234567
    To: Bob <sips:bob@biloxi.example.com>
    Call-ID: 12345600@atlanta.example.com
    CSeq: 1 INVITE
    Contact: <sips:a8342043f@atlanta.example.com;gr>
    Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
    Supported: replaces, gruu
    Content-Type: application/sdp
    Content-Length: [omitted]

    v=0
    o=alice 2890844526 2890844526 IN IP4 atlanta.example.com



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    s=
    c=IN IP4 atlanta.example.com
    t=0 0
    m=audio 49170 RTP/AVP 0
    a=rtpmap:0 PCMU/8000


    F2 180 Ringing Bob -> Alice

    SIP/2.0 180 Ringing
    Via: SIP/2.0/TLS atlanta.example.com:5061
     ;branch=z9hG4bK74bf9
     ;received=192.0.2.103
    From: Alice <sips:alice@atlanta.example.com>;tag=1234567
    To: Bob <sips:bob@biloxi.example.com>;tag=23431
    Call-ID: 12345600@atlanta.example.com
    CSeq: 1 INVITE
    Contact: <sips:bob@biloxi.example.com>
    Content-Length: 0


    F3 200 OK Bob -> Alice

    SIP/2.0 200 OK
    Via: SIP/2.0/TLS atlanta.example.com:5061
     ;branch=z9hG4bK74bf9
     ;received=192.0.2.103
    From: Alice <sips:alice@atlanta.example.com>;tag=1234567
    To: Bob <sips:bob@biloxi.example.com>;tag=23431
    Call-ID: 12345600@atlanta.example.com
    CSeq: 1 INVITE
    Contact: <sips:bob@biloxi.example.com>
    Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
    Supported: replaces
    Content-Type: application/sdp
    Content-Length: [omitted]

    v=0
    o=bob 2890844527 2890844527 IN IP4 biloxi.example.com
    s=
    c=IN IP4 biloxi.example.com
    t=0 0
    m=audio 3456 RTP/AVP 0
    a=rtpmap:0 PCMU/8000


    F4 ACK Alice -> Bob




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    ACK sips:bob@biloxi.example.com SIP/2.0
    Via: SIP/2.0/TLS atlanta.example.com:5061
     ;branch=z9hG4bK74bfd
    Max-Forwards: 70
    From: Alice <sips:alice@atlanta.example.com>;tag=1234567
    To: Bob <sips:bob@biloxi.example.com>;tag=23431
    Call-ID: 12345600@atlanta.example.com
    CSeq: 1 ACK
    Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
    Supported: replaces
    Content-Length: 0


    /* Bob places Alice on hold. */

    F5 INVITE Bob -> Alice

    INVITE sips:a8342043f@atlanta.example.com;gr SIP/2.0
    Via: SIP/2.0/TLS biloxi.example.com:5061
     ;branch=z9hG4bK874bk
    To: Alice <sips:alice@atlanta.example.com>;tag=1234567
    From: Bob <sips:bob@biloxi.example.com>;tag=23431
    Call-ID: 12345600@atlanta.example.com
    CSeq: 712 INVITE
    Contact: <sips:bob@biloxi.example.com>;+sip.rendering="no"
    Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
    Supported: replaces
    Content-Type: application/sdp
    Content-Length: [omitted]

    v=0
    o=bob 2890844527 2890844528 IN IP4 biloxi.example.com
    s=
    c=IN IP4 biloxi.example.com
    t=0 0
    m=audio 3456 RTP/AVP 0
    a=rtpmap:0 PCMU/8000
    a=sendonly


    F6 200 OK Alice -> Bob

    SIP/2.0 200 OK
    Via: SIP/2.0/TLS biloxi.example.com:5061
     ;branch=z9hG4bK874bk
     ;received=192.0.2.105
    To: Alice <sips:alice@atlanta.example.com>;tag=1234567
    From: Bob <sips:bob@biloxi.example.com>;tag=23431



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    Call-ID: 12345600@atlanta.example.com
    CSeq: 712 INVITE
    Contact: <sips:a8342043f@atlanta.example.com;gr>
    Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
    Supported: replaces, gruu
    Content-Type: application/sdp
    Content-Length: [omitted]

    v=0
    o=alice 2890844526 2890844527 IN IP4 atlanta.example.com
    s=
    c=IN IP4 atlanta.example.com
    t=0 0
    m=audio 49170 RTP/AVP 0
    a=rtpmap:0 PCMU/8000
    a=recvonly


    F7 ACK Bob -> Alice

    ACK sips:a8342043f@atlanta.example.com;gr SIP/2.0
    Via: SIP/2.0/TLS biloxi.example.com:5061
     ;branch=z9hG4bKq874b
    To: Alice <sips:alice@atlanta.example.com>;tag=1234567
    From: Bob <sips:bob@biloxi.example.com>;tag=23431
    Call-ID: 12345600@atlanta.example.com
    CSeq: 712 ACK
    Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
    Supported: replaces
    Content-Length: 0


    /* Bob's UA initiates music-on-hold. */

    F8 INVITE Bob -> Music Server

    INVITE sips:music@server.example.com SIP/2.0
    Via: SIP/2.0/TLS biloxi.example.com:5061
     ;branch=z9hG4bKnashds9
    Max-Forwards: 70
    From: Bob <sips:bob@biloxi.example.com>;tag=02134
    To: Music Server <sips:music@server.example.com>
    Call-ID: 4802029847@biloxi.example.com
    CSeq: 1 INVITE
    Contact: <sips:bob@biloxi.example.com>
    Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
    Supported: replaces, gruu
    Content-Type: application/sdp



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    Content-Length: [omitted]

    v=0
    o=bob 2890844534 2890844534 IN IP4 atlanta.example.com
    s=
    c=IN IP4 atlanta.example.com
    t=0 0
    m=audio 49170 RTP/AVP 0
    a=rtpmap:0 PCMU/8000
    a=recvonly


    F9 200 OK Music Server -> Bob

    SIP/2.0 200 OK
    Via: SIP/2.0/TLS biloxi.example.com:5061
     ;branch=z9hG4bKnashds9
     ;received=192.0.2.105
    From: Bob <sips:bob@biloxi.example.com>;tag=02134
    To: Music Server <sips:music@server.example.com>;tag=56323
    Call-ID: 4802029847@biloxi.example.com
    Contact: <sips:music@server.example.com>
    CSeq: 1 INVITE
    Content-Length: [omitted]

    v=0
    o=MusicServer 2890844576 2890844576 IN IP4 server.example.com
    s=
    c=IN IP4 server.example.com
    t=0 0
    m=audio 49170 RTP/AVP 0
    a=rtpmap:0 PCMU/8000
    a=sendonly


    F10 ACK Bob -> Music Server

    ACK sips:music@server.example.com SIP/2.0
    Via: SIP/2.0/TLS server.example.com:5061
     ;branch=z9hG4bK74bT6
    From: Bob <sips:bob@biloxi.example.com>;tag=02134
    To: Music Server <sips:music@server.example.com>;tag=56323
    Max-Forwards: 70
    Call-ID: 4802029847@biloxi.example.com
    CSeq: 1 ACK
    Content-Length: 0





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    /* Bob picks up the call by sending a re-INVITE to Alice. */

    F11 INVITE Bob -> Alice

    INVITE sips:a8342043f@atlanta.example.com;gr SIP/2.0
    Via: SIP/2.0/TLS biloxi.example.com:5061
     ;branch=z9hG4bK874bk
    To: Alice <sips:alice@atlanta.example.com>;tag=1234567
    From: Bob <sips:bob@biloxi.example.com>;tag=23431
    Call-ID: 12345600@atlanta.example.com
    CSeq: 713 INVITE
    Contact: <sips:bob@biloxi.example.com>
    Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
    Supported: replaces
    Content-Type: application/sdp
    Content-Length: [omitted]

    v=0
    o=bob 2890844527 2890844529 IN IP4 biloxi.example.com
    s=
    c=IN IP4 biloxi.example.com
    t=0 0
    m=audio 3456 RTP/AVP 0
    a=rtpmap:0 PCMU/8000


    F12 200 OK Alice -> Bob

    SIP/2.0 200 OK
    Via: SIP/2.0/TLS biloxi.example.com:5061
     ;branch=z9hG4bK874bk
     ;received=192.0.2.105
    To: Alice <sips:alice@atlanta.example.com>;tag=1234567
    From: Bob <sips:bob@biloxi.example.com>;tag=23431
    Call-ID: 12345600@atlanta.example.com
    CSeq: 713 INVITE
    Contact: <sips:a8342043f@atlanta.example.com;gr>
    Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
    Supported: replaces, gruu
    Content-Type: application/sdp
    Content-Length: [omitted]

    v=0
    o=alice 2890844526 2890844528 IN IP4 atlanta.example.com
    s=
    c=IN IP4 atlanta.example.com
    t=0 0
    m=audio 49170 RTP/AVP 0



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    a=rtpmap:0 PCMU/8000


    F13 ACK Bob -> Alice

    ACK sips:a8342043f@atlanta.example.com;gr SIP/2.0
    Via: SIP/2.0/TLS biloxi.example.com:5061
     ;branch=z9hG4bKq874b
    To: Alice <sips:alice@atlanta.example.com>;tag=1234567
    From: Bob <sips:bob@biloxi.example.com>;tag=23431
    Call-ID: 12345600@atlanta.example.com
    CSeq: 713 ACK
    Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
    Supported: replaces
    Content-Length: 0


    F14 BYE Bob -> Music Server

    INVITE sips:music@server.example.com SIP/2.0
    Via: SIP/2.0/TLS biloxi.example.com:5061
     ;branch=z9hG4bK74rf
    Max-Forwards: 70
    From: Bob <sips:bob@biloxi.example.com>;tag=02134
    To: Music Server <sips:music@server.example.com>;tag=56323
    Call-ID: 4802029847@biloxi.example.com
    CSeq: 2 INVITE
    Contact: <sips:bob@biloxi.example.com>
    Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
    Supported: replaces, gruu
    Content-Length: [omitted]


    F15 200 OK Music Server -> Alice

    SIP/2.0 200 OK
    Via: SIP/2.0/TLS atlanta.example.com:5061
     ;branch=z9hG4bK74rf
     ;received=192.0.2.103
    From: Bob <sips:bob@biloxi.example.com>;tag=02134
    To: Music Server <sips:music@server.example.com>;tag=56323
    Call-ID: 4802029847@biloxi.example.com
    CSeq: 2 BYE
    Content-Length: 0


    /* Normal media session between Alice and Bob is resumed */




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2.4.  Alternative Example Message Flow

   A disadvantage of the previous message flow is that the RTP is sent
   to the remote UA from an address that is different from that which is
   given in the offer SDP of the re-INVITE.  This can trigger SPIT-
   prevention behavior in some UA's.[Section 4] This section shows an
   alternative example message flow which avoids this problem.

   Again, this is just one possible message flow that illustrates this
   technique; numerous variations on these operations are allowed by the
   applicable standards.

   This technique involves interleaving the SDP offer/answer sequences
   of the two dialogs (original call and MOH):

   1.  The executing UA sends the re-INVITE without SDP, forcing the
       remote UA to provide an SDP offer in the 200 response.

   2.  The executing UA sends this offer as the SDP of the INVITE that
       it sends to the music-on-hold server.  (The offer is modified to
       have a suitable o= line, and the media directionality is set to
       "recvonly", the reverse of "on-hold".)

   3.  The music-on-hold server provides an SDP answer, which (we
       assume) includes its media source address as its listening
       address.

   4.  The executing UA sends this SDP answer as its SDP answer in the
       ACK for the re-INVITE.  (The answer is modified to have a
       suitable o= line.)


   Alice             Bob       Music Server

   Alice establishes the call:

     |                |              |
     |    INVITE F1   |              |
     |--------------->|              |
     | 180 Ringing F2 |              |
     |<---------------|              |
     |    200 OK F3   |              |
     |<---------------|              |
     |     ACK F4     |              |
     |--------------->|              |
     |       RTP      |              |
     |<==============>|              |
     |                |              |



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   Bob places Alice on hold, compelling Alice's UA to provide SDP:

     |                |              |
     |   INVITE F5    |              |
     |   (no SDP)     |              |
     |<---------------|              |
     |   200 OK F6    |              |
     |   (SDP offer)  |              |
     |--------------->|              |
     |                |              |

   Bob's UA initiates music-on-hold:

     |                |              |
     |                |  INVITE F7   |
     |                |  (SDP offer, |
     |                |   rev. hold) |
     |                |------------->|
     |                | 200 OK F8    |
     |                | (SDP answer, |
     |                |  hold)       |
     |                |<-------------|
     |                |    ACK F9    |
     |                |------------->|
     |                |              |

   Bob's UA provides SDP answer containing the address/port
   of the Music Server:

     |                |              |
     | ACK (hold) F10 |              |
     | (SDP answer)   |              |
     |<---------------|              |
     |    no RTP      |              |
     |                |              |
     |     Music-on-hold RTP         |
     |<==============================|
     |                |              |

   The music on hold is active.

   Bob takes Alice off hold:

     |                |              |
     |  INVITE F11    |              |
     |  (SDP offer)   |              |
     |<---------------|              |
     |   200 OK F12   |              |



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     |   (SDP answer) |              |
     |--------------->|              |
     |     ACK F13    |              |
     |<---------------|              |
     |                |    BYE F14   |
     |                |------------->|
     |                |    200 F15   |
     |                |<-------------|
     |       RTP      |              |
     |<==============>|              |
     |                |              |

   The normal media session between Alice and Bob is resumed.

   Message Details

    /* Alice calls Bob. */

    F1 INVITE Alice -> Bob

    INVITE sips:bob@biloxi.example.com SIP/2.0
    Via: SIP/2.0/TLS atlanta.example.com:5061
     ;branch=z9hG4bK74bf9
    Max-Forwards: 70
    From: Alice <sips:alice@atlanta.example.com>;tag=1234567
    To: Bob <sips:bob@biloxi.example.com>
    Call-ID: 12345600@atlanta.example.com
    CSeq: 1 INVITE
    Contact: <sips:a8342043f@atlanta.example.com;gr>
    Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
    Supported: replaces, gruu
    Content-Type: application/sdp
    Content-Length: [omitted]

    v=0
    o=alice 2890844526 2890844526 IN IP4 atlanta.example.com
    s=
    c=IN IP4 atlanta.example.com
    t=0 0
    m=audio 49170 RTP/AVP 0
    a=rtpmap:0 PCMU/8000


    F2 180 Ringing Bob -> Alice

    SIP/2.0 180 Ringing
    Via: SIP/2.0/TLS atlanta.example.com:5061
     ;branch=z9hG4bK74bf9



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     ;received=192.0.2.103
    From: Alice <sips:alice@atlanta.example.com>;tag=1234567
    To: Bob <sips:bob@biloxi.example.com>;tag=23431
    Call-ID: 12345600@atlanta.example.com
    CSeq: 1 INVITE
    Contact: <sips:bob@biloxi.example.com>
    Content-Length: 0


    F3 200 OK Bob -> Alice

    SIP/2.0 200 OK
    Via: SIP/2.0/TLS atlanta.example.com:5061
     ;branch=z9hG4bK74bf9
     ;received=192.0.2.103
    From: Alice <sips:alice@atlanta.example.com>;tag=1234567
    To: Bob <sips:bob@biloxi.example.com>;tag=23431
    Call-ID: 12345600@atlanta.example.com
    CSeq: 1 INVITE
    Contact: <sips:bob@biloxi.example.com>
    Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
    Supported: replaces
    Content-Type: application/sdp
    Content-Length: [omitted]

    v=0
    o=bob 2890844527 2890844527 IN IP4 biloxi.example.com
    s=
    c=IN IP4 biloxi.example.com
    t=0 0
    m=audio 3456 RTP/AVP 0
    a=rtpmap:0 PCMU/8000


    F4 ACK Alice -> Bob

    ACK sips:bob@biloxi.example.com SIP/2.0
    Via: SIP/2.0/TLS atlanta.example.com:5061
     ;branch=z9hG4bK74bfd
    Max-Forwards: 70
    From: Alice <sips:alice@atlanta.example.com>;tag=1234567
    To: Bob <sips:bob@biloxi.example.com>;tag=23431
    Call-ID: 12345600@atlanta.example.com
    CSeq: 1 ACK
    Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
    Supported: replaces
    Content-Length: 0




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    /* Bob places Alice on hold. */

    /* The re-INVITE contains no SDP, thus compelling Alice's UA
       to provide an offer. */

    F5 INVITE Bob -> Alice

    INVITE sips:a8342043f@atlanta.example.com;gr SIP/2.0
    Via: SIP/2.0/TLS biloxi.example.com:5061
     ;branch=z9hG4bK874bk
    To: Alice <sips:alice@atlanta.example.com>;tag=1234567
    From: Bob <sips:bob@biloxi.example.com>;tag=23431
    Call-ID: 12345600@atlanta.example.com
    CSeq: 712 INVITE
    Contact: <sips:bob@biloxi.example.com>;+sip.rendering="no"
    Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
    Supported: replaces
    Content-Length: 0

    /* Alice's UA provides an SDP offer.
       Since it does not know that it is being put on hold,
       the offer is the same as the original offer and describes
       bidirectional media. */

    F6 200 OK Alice -> Bob

    SIP/2.0 200 OK
    Via: SIP/2.0/TLS biloxi.example.com:5061
     ;branch=z9hG4bK874bk
     ;received=192.0.2.105
    To: Alice <sips:alice@atlanta.example.com>;tag=1234567
    From: Bob <sips:bob@biloxi.example.com>;tag=23431
    Call-ID: 12345600@atlanta.example.com
    CSeq: 712 INVITE
    Contact: <sips:a8342043f@atlanta.example.com;gr>
    Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
    Supported: replaces, gruu
    Content-Type: application/sdp
    Content-Length: [omitted]

    v=0
    o=alice 2890844526 2890844526 IN IP4 atlanta.example.com
    s=
    c=IN IP4 atlanta.example.com
    t=0 0
    m=audio 49170 RTP/AVP 0
    a=rtpmap:0 PCMU/8000
    a=active



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    /* Bob's UA initiates music-on-hold. */

    /* This INVITE contains Alice's offer, but with the media
       direction set to "reverse hold", receive-only. */

    F7 INVITE Bob -> Music Server

    INVITE sips:music@server.example.com SIP/2.0
    Via: SIP/2.0/TLS biloxi.example.com:5061
     ;branch=z9hG4bKnashds9
    Max-Forwards: 70
    From: Bob <sips:bob@biloxi.example.com>;tag=02134
    To: Music Server <sips:music@server.example.com>
    Call-ID: 4802029847@biloxi.example.com
    CSeq: 1 INVITE
    Contact: <sips:bob@biloxi.example.com>
    Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
    Supported: replaces, gruu
    Content-Type: application/sdp
    Content-Length: [omitted]

    v=0
    o=bob 2890844534 2890844534 IN IP4 atlanta.example.com
    s=
    c=IN IP4 atlanta.example.com
    t=0 0
    m=audio 49170 RTP/AVP 0
    a=rtpmap:0 PCMU/8000
    a=recvonly


    F8 200 OK Music Server -> Bob

    SIP/2.0 200 OK
    Via: SIP/2.0/TLS biloxi.example.com:5061
     ;branch=z9hG4bKnashds9
     ;received=192.0.2.105
    From: Bob <sips:bob@biloxi.example.com>;tag=02134
    To: Music Server <sips:music@server.example.com>;tag=56323
    Call-ID: 4802029847@biloxi.example.com
    Contact: <sips:music@server.example.com>
    CSeq: 1 INVITE
    Content-Length: [omitted]

    v=0
    o=MusicServer 2890844576 2890844576 IN IP4 server.example.com
    s=
    c=IN IP4 server.example.com



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    t=0 0
    m=audio 49170 RTP/AVP 0
    a=rtpmap:0 PCMU/8000
    a=sendonly


    F9 ACK Bob -> Music Server

    ACK sips:music@server.example.com SIP/2.0
    Via: SIP/2.0/TLS server.example.com:5061
     ;branch=z9hG4bK74bT6
    From: Bob <sips:bob@biloxi.example.com>;tag=02134
    To: Music Server <sips:music@server.example.com>;tag=56323
    Max-Forwards: 70
    Call-ID: 4802029847@biloxi.example.com
    CSeq: 1 ACK
    Content-Length: 0


    /* Bob's UA now sends the ACK that completes the re-INVITE
       to Alice and completes the SDP offer/answer.
       The ACK contains the SDP received from the Music Server,
       and thus contains the address/port from which the Music Server
       will send media. */

    F10 ACK Bob -> Alice

    ACK sips:a8342043f@atlanta.example.com;gr SIP/2.0
    Via: SIP/2.0/TLS biloxi.example.com:5061
     ;branch=z9hG4bKq874b
    To: Alice <sips:alice@atlanta.example.com>;tag=1234567
    From: Bob <sips:bob@biloxi.example.com>;tag=23431
    Call-ID: 12345600@atlanta.example.com
    CSeq: 712 ACK
    Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
    Supported: replaces
    Content-Length: [omitted]

    v=0
    o=bob 2890844527 2890844528 IN IP4 biloxi.example.com
    s=
    c=IN IP4 server.example.com
    t=0 0
    m=audio 49170 RTP/AVP 0
    a=rtpmap:0 PCMU/8000
    a=sendonly

    /* Bob picks up the call by sending a re-INVITE to Alice. */



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    F11 INVITE Bob -> Alice

    INVITE sips:a8342043f@atlanta.example.com;gr SIP/2.0
    Via: SIP/2.0/TLS biloxi.example.com:5061
     ;branch=z9hG4bK874bk
    To: Alice <sips:alice@atlanta.example.com>;tag=1234567
    From: Bob <sips:bob@biloxi.example.com>;tag=23431
    Call-ID: 12345600@atlanta.example.com
    CSeq: 713 INVITE
    Contact: <sips:bob@biloxi.example.com>
    Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
    Supported: replaces
    Content-Type: application/sdp
    Content-Length: [omitted]

    v=0
    o=bob 2890844527 2890844529 IN IP4 biloxi.example.com
    s=
    c=IN IP4 biloxi.example.com
    t=0 0
    m=audio 3456 RTP/AVP 0
    a=rtpmap:0 PCMU/8000


    F12 200 OK Alice -> Bob

    SIP/2.0 200 OK
    Via: SIP/2.0/TLS biloxi.example.com:5061
     ;branch=z9hG4bK874bk
     ;received=192.0.2.105
    To: Alice <sips:alice@atlanta.example.com>;tag=1234567
    From: Bob <sips:bob@biloxi.example.com>;tag=23431
    Call-ID: 12345600@atlanta.example.com
    CSeq: 713 INVITE
    Contact: <sips:a8342043f@atlanta.example.com;gr>
    Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
    Supported: replaces, gruu
    Content-Type: application/sdp
    Content-Length: [omitted]

    v=0
    o=alice 2890844526 2890844527 IN IP4 atlanta.example.com
    s=
    c=IN IP4 atlanta.example.com
    t=0 0
    m=audio 49170 RTP/AVP 0
    a=rtpmap:0 PCMU/8000




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    F13 ACK Bob -> Alice

    ACK sips:a8342043f@atlanta.example.com;gr SIP/2.0
    Via: SIP/2.0/TLS biloxi.example.com:5061
     ;branch=z9hG4bKq874b
    To: Alice <sips:alice@atlanta.example.com>;tag=1234567
    From: Bob <sips:bob@biloxi.example.com>;tag=23431
    Call-ID: 12345600@atlanta.example.com
    CSeq: 713 ACK
    Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
    Supported: replaces
    Content-Length: 0


    F14 BYE Bob -> Music Server

    INVITE sips:music@server.example.com SIP/2.0
    Via: SIP/2.0/TLS biloxi.example.com:5061
     ;branch=z9hG4bK74rf
    Max-Forwards: 70
    From: Bob <sips:bob@biloxi.example.com>;tag=02134
    To: Music Server <sips:music@server.example.com>;tag=56323
    Call-ID: 4802029847@biloxi.example.com
    CSeq: 2 INVITE
    Contact: <sips:bob@biloxi.example.com>
    Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
    Supported: replaces, gruu
    Content-Length: [omitted]


    F15 200 OK Music Server -> Alice

    SIP/2.0 200 OK
    Via: SIP/2.0/TLS atlanta.example.com:5061
     ;branch=z9hG4bK74rf
     ;received=192.0.2.103
    From: Bob <sips:bob@biloxi.example.com>;tag=02134
    To: Music Server <sips:music@server.example.com>;tag=56323
    Call-ID: 4802029847@biloxi.example.com
    CSeq: 2 BYE
    Content-Length: 0


    /* Normal media session between Alice and Bob is resumed */







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3.  Advantages

   This technique for providing music-on-hold has advantages over other
   methods now in use:

   1.  The original dialog is not transferred to another UA, so the
       "remote endpoint URI" displayed by the remote endpoint does not
       change during the call.[3]

   2.  The music-on-hold media is sent directly from the music-on-hold
       source to the remote UA, rather than being relayed through the
       holding UA.

   3.  The technique does not require complex manipulation of SDP, and
       particularly does not require a SIP agent to modify received SDP
       to be acceptable to be sent within an already established
       sequence of SDP (which can require a complex accounting of the m=
       lines).

































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4.  Security Considerations

   SDP, by its organization, specifies what address and port a UA will
   use to listen for media, but implicitly allows media to be sent to
   that address and port from any address and port.  Some UAs, in order
   to avoid SPIT, will refuse to render media that are sent from an
   address which is not the listening address for the remote UA.  That
   policy will also block music-on-hold that is provided using this
   technique.

   This problem can be circumvented in at least three ways:

   The first method is to disable the media-address restriction.  Since
   this brings the UA's behavior into alignment with the common SIP
   model, it is probably the most reliable for overall interoperability.

   The second method is for the holding UA to obtain knowledge of the
   media sending address of the music-on-hold server by some means, and
   substitute that address for its own in the SDP in the re-INVITE that
   places the dialog on-hold.  This technique requires that the holding
   UA can obtain this sending address, and that the technique for
   specifying the "on-hold" condition in the re-INVITE is not that of
   setting the listening address in the c= line to 0.0.0.0.

   The third method is to use the more complex interleaved SDP offer/
   answer system illustrated in Section 2.4.  This technique requires
   that the executing UA can coordinate the SDP offer/answer mechanism
   between two dialogs.























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5.  References

5.1.  Normative References

   [1]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
        Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
        Session Initiation Protocol", RFC 3261, June 2002.

   [2]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
        the Session Description Protocol (SDP)", RFC 3264, June 2002.

5.2.  Informative References

   [3]  Johnston, A., Sparks, R., Cunningham, C., Donovan, S., and K.
        Summers, "Session Initiation Protocol Service Examples",
        I-D draft-ietf-sipping-service-examples-13, July 2007.



































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Author's Address

   Dale R. Worley
   Bluesocket Inc.
   10 North Ave.
   Burlington, MA  01803
   US

   Phone: +1 781 229 0533 x173
   Email: dworley@pingtel.com
   URI:   http://www.pingtel.com








































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Intellectual Property Statement

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Disclaimer of Validity

   This document and the information contained herein are provided on an
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   THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
   OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
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Copyright Statement

   Copyright (C) The IETF Trust (2007).  This document is subject to the
   rights, licenses and restrictions contained in BCP 78, and except as
   set forth therein, the authors retain all their rights.


Acknowledgment

   Funding for the RFC Editor function is currently provided by the
   Internet Society.




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